1. The field of the Invention
The present invention is directed to a power supply, and more particularly to an inverter AC power supply which is connected to an commercial ac voltage source to provide therefrom through an ac-dc-ac conversion a high frequency AC voltage to a load with a maximum efficiency.
2. Description of the Prior Art
Inverter AC power supplies are known to comprise an ac-to-dc converter providing a dc voltage form a commercial ac source voltage and an inverter providing from the dc voltage a high frequency voltage for driving loads such as discharge lamps. It is also known to utilizes a chopper in the circuit which operates to chop the ac source voltage in providing the dc voltage to the inverter input for reducing the size of an inductor element necessary for improving a power factor of the circuit. In order to avoid duplication of components for the chopper and inverter, it has been proposed in the preceding U.S. application Ser. No. 407,093 now U.S. Pat. No. 4,933,831, to share switching elements for the chopper and inverter circuits. Although such a prior power supply is found most effective in reducing the number of circuit components while assuring an improved power factor, it is rather difficult to control an input power W.sub.IN to the chopper and an output power W.sub.OUT from the inverter independently of one another due to the limitation that the switching elements are common to the chopper and the inverter. This poses another problem when the power supply suffers from considerable variations in the input power W.sub.IN or output power W.sub.OUT which may result from, for example, varying load conditions, fluctuations in the ac source voltage, or other factors. That is, when the input power to the chopper is for some reason reduced to have W.sub.IN &lt;W.sub.OUT, considerable harmonics will appear, in an input current from the AC source thereby causing undesirable input distortion and eventually reducing the power factor. On the other hand, when the input power to the chopper is increased to have W.sub.IN &gt;W.sub.OUT, the chopper output is correspondingly increased so as to apply such increased voltage to the switching elements and smoothing capacitors forming the chopper and the inverter. Consequently, it is required to utilize the switching element and the capacitors which can withstand such increased voltage for safe operation of the circuit. However, such components are unexceptionably expensive and will certainly add extra cost to the power supply. Therefore, it is highly demanded to positively control for balancing the chopper input power W.sub.IN and the inverter output power W.sub.OUT, particularly in the power supply circuit utilizing the switching element common to the chopper and the inverter for the purpose of reducing the input distortion to a minimum, maintaining improved power factor, and preventing an undue increase in cost.